The invention relates to a DC/DC converter including a charge pump circuit and a method of operating one such DC/DC converter.
In addition to the supply voltage many electronic circuits require further voltages, the levels of which are sometimes higher than that of the supply voltage. One cost-effective, simple andxe2x80x94especially as compared to coil convertersxe2x80x94highly compact solution to furnishing these further voltages are DC/DC converters operating on the charge pump principle. Such converters are described e.g. in the text book xe2x80x9cThe Art of Electronicsxe2x80x9d by Paul Horowitz, 2nd Edition, Cambridge University Press, New York 1991 on pages 377 to 379 thereof.
Horowitz also describes a simple DC/DC converter operating on the charge pump principle with which an output voltage corresponding maximally to roughly twice the input voltage is achievable. The basic circuit of the converter consists substantially of a charge pump capacitor and four controllable switches (e.g. MOSFETS) whereby one electrode of the charge pump capacitor is connectable via a first switch to the input voltage terminal of the converter and via a second switch to GND, and the other electrode of the capacitor is connectable via the third switch to the input voltage terminal and via the fourth switch to the output voltage terminal of the converter. The converter comprises furthermore a control circuit including a clock which clocks the switches so that in a first phase of a clock cycle, the so-called charging phase, the second switch and the third switch are ON whilst the other switches are OFF, so that the charge pump capacitor is charged to, the input voltage, and in the second phase of a clock cycle, the so-called discharge phase, the first switch and the fourth switch are ON whilst the other switches are OFF, so that then the charged charge pump capacitor is connected in series to the input voltage which outputs a voltage value to the smoothing and storage capacitor located at the output of the circuit, this voltage value corresponding to roughly twice the input voltage.
Correspondingly, charge pumps are conceivable which produce an optimum multiple of the input voltage, which invert or reduce the input voltage. However, in the DC/DC converter operating on the charge pump principle as described above the output voltage drops off undesirably so even for small load currents. Since in the majority of applications the output voltage which e.g. in digital electronic circuits amounts often to 3.3 or 5 V, is fixedly defined and is only allowed to fluctuate in a tight range, regulated converters have been developed which set the output voltage to a fixed desired voltage value.
These DC/DC converter regulators comprise as a rule a comparator which compares the actual output voltage or a voltage proportional to the actual output voltage (which may be derived from the output voltage e.g. across a voltage divider) to a defined reference voltage representing the design output voltage, and then when a deviation is sensed, outputs a control signal, with the aid of which the actual output voltage is adapted to the defined design output voltage value.
Described in U.S. Pat. No. 5,680,300 are two types of regulators used with DC/DC converters operating on the charge pump principle, the so-called linear regulator and the so-called skip-mode regulator.
In the linear regulator the control signal of the comparator changes, e.g. via a, gate of one of the MOSFET switches, the ON resistance of the MOSFET so that the drop in voltage across the switch is increased or decreased resulting in a reduction or increase in the actual output voltage of the converter. The linear regulator has, however, the disadvantage that the energy losses resulting from switching the switches of the charge pump are relatively high since the charge pump is always in operation in the case of the linear regulator. These energy losses result from the currents required to charge the gates of the MOS power transistors at a constant frequency even when no current flows at the output of the converter.
These disadvantage do not occur in the skip-mode regulator which makes use of the control signal of the comparator to cycle the charge pump ON/OFF depending on the output current requirement and the resulting actual output voltage of the DC/DC converter so that a charge is pumped to the smoothing and storage capacitor located at the output of the circuit only if the voltage across the capacitor has dropped below the design output voltage level. The skip-mode regulator thus operates particularly energy-saving and is particularly suitable for applications in which small load currents alternate with large load currents, i.e. it guaranteeing a minor quiescent current of the converter. The disadvantage of the skip-mode regulator is, however, that the ON/OFF switching times of the charge pump depend on the average output current in each case to be furnished by the DC/DC converter, i.e. the frequency spectrum resulting at the output of the converter is totally undefined. In addition to this the ripple of the output voltage is relatively heavy since the flow of output current is not continual in the skip-mode regulator.
A general object of a general present invention is thus to provide a DC/DC converter operating on the charge pump principle which is superior to the DC/DC converters regulated hitherto either by the skip mode or linear regulator principle and obviates the disadvantages as described above at least in part. In addition, the intention is to provide a corresponding method of operating a DC/DC converter which is superior to the method hitherto.
This and other objects and features are achieved in accordance with one aspect of the invention by a DC/DC converter including a charge pump circuit comprising:
one or more charge pump capacitors and a plurality of controllable switches connected thereto, the controllable switches being controllable by a control circuit so that the charge pump capacitors is/are alternatingly switched in a charging and discharge phase so that an output voltage deviating from the input voltage of the converter is generated at the output of the converter;
a first current source set to a predetermined base current located either in the discharge path of the charge pump circuit via which in the discharge phase current is supplied to the output of the converter, or in the charging path of the charge pump circuit, via which the charge pump capacitors is/are charged in the charging phase of the charge pump circuit; and
a second current source connected in parallel to the first current source, the current of the second current source being controllable; and
an output voltage regulator circuit for generating a first control signal representing the difference between a voltage characterizing the output voltage and a first reference voltage and controlling the second current source when the charge pump circuit is active so that the controllable current is reduced or increased with an increase and reduction respectively in the difference to track the voltage characterizing the output voltage in accordance with the first reference voltage; and for generating a second control signal guided to the control circuit, this signal assuming a first status when the voltage characterizing the output voltage exceeds a second reference voltage at a predetermined level above the- first reference voltage, upon which the control circuit deactivates the charge pump circuit, and assumes a second status when the voltage characterizing the output voltage drops below the second reference voltage, upon which the control circuit activates the charge pump circuit.
Another aspect of the invention includes a method for operating a DC/DC converter including a charge pump circuit comprising one or more charge pump capacitors and a plurality of controllable switches connected thereto comprising the steps cycling the charge pump capacitors by the controllable switches in a charging and discharge phase during operation of the charge pump circuit so that an output voltage deviating from the input voltage of the converter is generated at the output of the converter;
setting a controllable current flowing parallel to a predetermined base current with the charge pump circuit active in the discharge or charging path of the charge pump circuit as a function of the difference between a voltage characterizing the output voltage and a first reference voltage so that the controllable current is reduced or increased with an increase and reduction respectively in the difference to track the voltage characterizing the output voltage in accordance with the first reference voltage; and deactivating the charge pump circuit when the voltage characterizing the output voltage exceeds a second reference voltage at a predetermined level above the first reference voltage and activating the charge pump circuit when the voltage characterizing the output voltage drops below the second reference voltage at a predetermined level above the first reference voltage.
The DC/DC converter in accordance with, the invention makes clever use of the advantages afforded by the two differing regulated converters by it being skip-mode regulated when the converter output current is low and linearly regulated when the converter output current is higher, selecting the one or other regulating mode being done automatically and simply achievable. The DC/DC converter in accordance with the invention comprises both a high efficiency at low output currents and a defined output frequency spectrum at high output currents.
Advantageous further embodiments of the invention are characterized in the sub-claims.